The study of the role of experience in vocal ontogeny in birds has been extremely influential on theories of human development, but few studies have demonstrated experiential influences in mammalian vocal development. Recent studies in birds and mammals have suggested a new form of socially- based vocal learning that appears in wide range of taxa, including nonhuman primates and humans. Three model systems are proposed that use pygmy marmosets and cotton-top tamarins to study the ontogeny of vocal production and vocal usage. 1. Food-associated Calls: These calls are produced when animals encounter a preferred food. Several studies are proposed to examine the function of these calls: Do they attract other group members or prevent others from approaching the food? Does the presence and nature of an audience (potential rivals versus infants and nursing females) influence call production? How are calls used to assist infants during weaning transitions as they encounter solid foods? Do infants have an adult-like structure and usage at the first appearance of these calls or is there a developmental progression in response to social cues from other group members? Do dominant animals within a group suppress the adult-like structure and usage of calls? 2. Infant Babbling: Babbling has been hypothesized to be of critical importance in human vocal development and babbling has parallels in the subsong and plastic song of young birds. Both species of monkeys show vocal babbling at the same relative developmental stage as human infants. Several studies are proposed to examine the frequency of occurrence, the behavioral contexts in which babbling occurs, the fine structure of babbling vocalizations and the degree to which babbling might serve as vocal practice leading to more adult-like structure. The function of babbling will be studied by comparing animals with different rates of infant babbling on several social and reproductive parameters when they become adult. 3. Social Influences on Vocal Structure: Monkeys alter vocal structure when paired with a new mate and when other changes in social groups occur. We will examine the stability of vocal changes during the first year after pairing and whether paired monkeys, briefly separated from each other, adjust vocal structure to match changes in their partner's separation calls. The overall goal of this research is to develop non-human primate models that can supplement more traditional bird song models to gain a better understanding of the evolutionary and developmental influences in human vocal communication. Nonhuman primate vocalizations are much more similar to human vocalizations than bird song, and thus can aid in theory development and practical solutions to disorders in human language development.